1. Field of the Invention
This invention relates to an acceleration slip control device for a vehicle by which a slipping of the driving wheels of the vehicle during a start or acceleration of the vehicle is controlled.
2. Description of the Related Art
An acceleration slip control device is commonly used to improve the acceleration and straight line running stability of the vehicle, by preventing an excess slippage of the driving wheels during a start or acceleration of the vehicle.
Usually, the acceleration slip control device determines a target value for the speed of rotation of the driving wheels, and calculates the difference between the target value and the actual speed of rotation of the driving wheel (hereinafter called the deviation from the target speed). The acceleration slip control device controls the amount of slippage of the driving wheel so that the deviation from the target speed is maintained at less than a predetermined value.
To obtain a better response, a feedback control of the slippage is carried out by using a proportional integral and derivative control (PID control) or a proportional and derivative control (PD control).
For example, an acceleration slip control device in which a drive torque of the driving wheel is PID controlled, based on the slippage, is disclosed by Japanese Unexamined Patent Publication No. 64-7102.
It was found through experiment, however, that, although this type of control device based on a PID or PD control shows a good response and high stability on a flat road, the acceleration of the vehicle on a rough road is worsened because the drive torque is usually set lower by this type of control.
On a rough road, the rate of change of the slippage of the driving wheels varies much more than on a flat road, and since the rate of change of the slippage (i.e., derivative term) is a factor for determining the drive torque in the PID or PD control, it is considered that large variations of the rate of change of the slippage adversely affect the control of the drive torque and cause a small increase of the drive torque while greatly reducing the torque.